3He abundances; MORB vs OIB
In all variants of the standard model, OIB gases are considered to be the
residue of massive degassing
Degassed MORB has more 3He than OIB
Gonnermann &
Mukhopadhyay
with additions
mixing
[3He]
Loihi
MORB
3He/22Ne
MORB
OIB
W.Greenland
Baffin Bay
Seawater, air, trapped exsolved
gases, shallow mantle
Popping
rock
degassing
“the gap”
paradox
3He
In the Canonical Model OIB contains residual gas. In the
perisphere model OIB picks up old exsolved gases plus air-like
gas
Tolstikhin
carbonatites
High
3He/4He &
high 3He!
[3He]
HIMU
Note large
(103-104)
range
“plume-magmas”,
(e.g. Djibouti)
Loihi
air
Carbonatites have 3He/4He ratios that extend
from 0.04 to 39 Ra & average 9 +/- 14 Ra!
FOZO
3He/4He
Very
few
3He
atoms
“High”
R/Ra
basalts
extend
down to
MORB
HIMU
MORB
“Low” R/Ra
extend up to
MORB
Meibom
FOZO
3He/4He
Very
few
3He
atoms
“High”
R/Ra
basalts
extend
down to
MORB
HIMU
“Low” R/Ra
extend up to
MORB
THE VARIANCE
PARADOX; high R/Ra
are always associated
MORB
with low R/Ra
Unfortunately, maximum R/Ra values in Greenland,
Meibom
Samoa, Afar etc. are usually compared
with the
average MORB values
OIB are
identical 10-8
to MORB
(with
10-9
much
lower
3He)
Hypothetical undegassed OIB (no evidence for)
MORB
gap
High R/Ra
OIB
No
degassing
trends
Ozima & Igarashi 2000
There is no reason to
suppose & there never
has been that high
3He/4He means high
contamination
3He! OIB are from 3He
deficient sources
High 3He/4He can &
probably does mean low
4He, low U-Th
Distance from Mature Ridge
25
3He/
4He
(R/Ra)
8
25
3He
atoms
R/Ra of mix
Contribution from
MORB
OIB
R/Ra
Contribution from
ambient mantle
6
3He
atoms
MORB has higher 3He than OIB, even after
multiple stages of degassing. This is why
high 3He/4He is a midplate signature
Gonnermann &
Mukhopadhyay
with additions
mixing
Loihi
MORB
OIB
Popping
rock
degassing
Seawater, air, trapped exsolved
gases, shallow mantle
MORB, if
present, will
dominate
any mix
FOZO
Reason why
high R/Ra
samples are
midplate or
away from
mature rapidly
spreading
ridges
High 3He/4He,
low [3He]
component only
detectable away
from MORB
High [3He]
component
(MORB)
dominates
Change in
perspective; “plume
components” are
not due to presence
of plume but
absence of ridge
EM2
100
Anderson 1993
50
0
or FOZO/EM2
% MORB
In all variants of the standard model, OIB gases are
considered to be the residue of massive degassing
Degassed MORB has more 3He than OIB
Gonnermann &
Mukhopadhyay
with additions
mixing
[3He]
Loihi
MORB
3He/22Ne
MORB
OIB
Seawater, air, trapped exsolved
gases, shallow mantle
Popping
rock
degassing
“the gap”
paradox
3He
In the Canonical Model OIB contains residual gas. In the
perisphere model OIB picks up old exsolved gases plus air-like
gas
MORB has higher 3He than OIB, even after
multiple stages of degassing. This is why
high 3He/4He is a midplate signature
Gonnermann &
Mukhopadhyay
with additions
mixing
MORB
MORB
OIB
Popping
rock
degassing
Seawater, air, trapped exsolved
gases, shallow mantle
Ambient or hotspot mantle
GAP
MORB, if
present, will
dominate
any mix
OIB is not extensively degassed; it is moderately
Carbonatites: slab components?
contaminated
MORB
Residual
gases
Degassed
gases
OIB
vesicles
Atmospheric/seawater
contamination
Secondary
trapped He
MORB is gas rich!
High levels
of 3He
[3He]
Popping rock
MORB
Most
OIB
Most continental “plumes”
Because of their
high 3He
Kola peninsula
concentrations,
kimberlites
MORB
& some &
carbonatites
carbonatites &
dominate
any
inclusions
mixing
U=18 to 0.1 ppm
Mantle
xenoliths in
carbonatites
can have
high R/Ra
and [3He]
0.07
These are
attributed to
plumes but they
are part of a
continuum
R/Ra
7
24
It is not true,
as often
asserted that
all xenoliths
are ~< 8 R/Ra
These are grab samples from the
shallow mantle
Tolstikhin
Highest 3He
Highest
3He/4He
MORB
Highest 3He materials
on Earth: popping rock,
MORB, carbonatites,
manganese nodules
OIB
Hypothetical
undegassed
reservoir